Understanding the phenotypic diversity of interneurons in the cerebral cortex is critical because interneuron dysfunction has been implicated in neurological disorders. Numerous subgroups of cortical interneurons have been identified. The current proposal extends from my first AREA grant and focuses on an enigmatic subset of interneurons that expresses tyrosine hydroxylase (TH), the rate-limiting catecholaminergic enzyme, but no subsequent enzymes in this family. Little is known about cortical TH neurons despite the fact that they are lost in Parkinson's disease and dementia. During postnatal rat development, the number of cortical TH-immunoreactive (IR) neurons sharply increases and then decreases. This reduction is not due to cell death but due to a decline in the detectable level of TH in these cells as they acquire their mature phenotype. A subset of these cells colocalizes with the GABA synthetic enzyme GAD and with calretinin, present in one of three distinct interneuron subpopulations. The proposed studies address important unresolved questions. First, I will immunohistochemically characterize the neurotransmitter phenotype of cortical TH-IR neurons in developing and adult rats. The activity of TH will be determined by examining the phosphorylation of TH at Ser19, 31, and 40, where phosphorylation regulates activity. The developmental coexpression of TH with GABA, vasoactive intestinal peptide (VIP), and the cholinergic enzyme choline acetyltransferase (ChAT) will be assessed. Second, I will characterize the neurochemical phenotype of TH-IR neurons in the adult human cortex. Half of these cells reportedly are GABAergic, but few other characteristics have been examined. The activity of TH will be addressed by examining its phosphorylation in human cortical neurons. The coexpression of TH with GABA, VIP, and ChAT and with the calcium-binding proteins calretinin, calbindin, and parvalbumin will be assessed. The neurochemical characterization of cortical TH cells in rats and humans will increase our understanding of the function of these cells across species. The proposed studies will provide opportunities for undergraduates at a liberal arts college to engage in clinically-relevant biomedical research, encouraging their pursuit of postgraduate training in the biomedical sciences. PUBLIC HEALTH RELEVANCE: Interneurons of the cerebral cortex play a key role in cognitive function, and interneuron abnormalities have been implicated in neurological disorders. The proposed studies explore the chemical properties of interneurons that express tyrosine hydroxylase, an enzyme that produces the dopamine family of neurotransmitters, in the rat and human brain with the goal of better understanding their function and role in disease.